The present invention relates to an improved method for removing hydrogen sulfide from gases. More specifically, the present invention relates to an improvement in a method for removing hydrogen sulfide from produced natural gas with iron oxide.
Produced fuel gases such as natural gas and petroleum light gas fractions often contain substantial concentrations of hydrogen sulfide. Normally, the hydrogen sulfide must be removed before the produced gas is suitable for sale or distribution through pipelines, in order to meet the product specifications required by commercial gas suppliers. It is also desirable to remove hydrogen sulfide from fuel gases before they are burned to reduce sulfur oxides emissions. In addition to produced fuel gases, other gases, such as petroleum refinery off-gas streams, are often contaminated with hydrogen sulfide. The presence of hydrogen sulfide can be detrimental when such gases are used in hydrocarbon or petrochemical processing. The presence of hydrogen sulfide also complicates the disposal of refinery gases by, for example, flaring, since the sulfur dioxide produced during burning poses an emissions problem.
Various techniques have been suggested for removing hydrogen sulfide from fuel and other gases. The term "acid gases" include carbon dioxide, as well as hydrogen sulfide, since both these compounds can dissolve in water to form corrosive, acidic solutions. In many cases, conventional purification treatment of gas streams, such as fuel gas streams, has as its purpose the removal of the acid gas components, not just hydrogen sulfide. In such cases, removal of substantial amounts of carbon dioxide in addition to removal of hydrogen sulfide is desirable. On the other hand, produced hydrocarbon gases such as natural gas may contain a substantial amount of carbon dioxide, which is not detrimental to the commercial value of the natural gas. Gas purification processes which remove carbon dioxide from a gas, as well as hydrogen sulfide, may therefore be needlessly complicated and expensive for use in purifying natural gas, and can reduce the volume of natural gas available for sale or distribution. Accordingly, it is sometimes advantageous to remove hydrogen sulfide from natural gas and other produced fuel gases without substantially changing the concentration of carbon dioxide in the gas.
Among the methods proposed for the removal of hydrogen sulfide from gases has been the use of solid, dry materials such as zeolites or iron sponge. The use and regeneration of zeolites is often impractical for processing large quantities of relatively inexpensive natural gas, and zeolites usually remove carbon dioxide as well as hydrogen sulfide. Another technique used for removing hydrogen sulfide from gases has been scrubbing the gases with an aqueous or other liquid solution. Generally, the liquid materials such as aqueous solutions of amines, alkanolamines, potassium carbonate and the like, which have been used to remove hydrogen sulfide from gases by liquid phase scrubbing also remove substantial amounts of carbon dioxide. These aqueous and other liquid aqueous scrubbing processes usually operate by forming a chemical addition product from hydrogen sulfide and, for example, an amine in liquid solution and then stripping the hydrogen sulfide out of the liquid solution in concentrated form. Disposal of this concentrated hydrogen sulfide usually requires a Claus unit, or the like, for converting the hydrogen sulfide to sulfur. Some types of liquid scrubbing-type purification processes actually require a substantial concentration of carbon dioxide in the gas to be treated for operability in desulfurization. Various other solvents, which may be termed physical solvents for hydrogen sulfide, have also been employed or suggested. Generally, these physical solvents simultaneously remove carbon dioxide from the gas, in addition to hydrogen sulfide. Direct conversion of hydrogen sulfide to sulfur in aqueous or other liquid solutions has also been proposed. For example, it has been suggested to dissolve hydrogen sulfide in an alkaline solution and to oxidize the hydrogen sulfide in the liquid solution with one of a variety of oxidation catalysts to provide elemental sulfur in the liquid.
Removal of hydrogen sulfide from gases by contacting them with iron oxide has long been known, and is commercially practiced. Iron oxide has been used in the form of pellets, powder, etc., and has been used on essentially inert solid supports such as wood shavings, clays, etc. Iron oxide on wood shavings, known as iron sponge, is used commercially for removing hydrogen sulfide from produced fuel gases such as natural gas. Iron sponge desulfurization has the advantages of simplicity, economy and ability to remove hydrogen sulfide selectively without affecting carbon dioxide in the gas. Iron sponge desulfurization is sometimes called the "dry-box" process. In natural gas purification, the gas is passed through a bed of iron sponge. Hydrogen sulfide reacts with iron oxide (e.g., ferric oxide) to form iron sulfide (e.g., ferric sulfide). Enough air is added to the natural gas to provide molecular oxygen to react with the iron sulfide to regenerate the iron oxide and form elemental sulfur. The bed of iron sponge can be used until it becomes plugged or deactivated with elemental sulfur. The deactivated bed is then removed from the purification vessel and discarded. Alkali metal salt solutions may be added to the iron sponge bed periodically to increase its efficiency.
It is often difficult to remove a spent bed of iron sponge from the purification vessel, since the bed tends to harden into a cohesive mass resistant to convenient removal means such as water spraying. Disposal of the spent sponge is often complicated by a sulfurous odor present in the spent sponge. The improvement of the present invention is directed, in part, to overcoming these and other drawbacks in conventional iron-sponge gas-purification methods.